1. Field of the Invention
The present invention relates to a spindle motor and a rotation apparatus employing a hydrodynamic bearing.
2. Description of the Related Art
More and more hydrodynamic bearing devices are replacing ball bearing devices which have been conventionally used, as bearing devices used in spindle motors in hard discs, polygon mirrors, optical disc apparatuses and the like. The hydrodynamic bearing devices are superior to the ball bearings in a rotational accuracy and silent property. Also, more spindle motors are used in mobile computing devices. Thus, there is a need for further miniaturization of the spindle motors.
Japanese Laid-Open Publication No. 2004-19705 proposes a structure shown in FIGS. 9 and 10 as a bearing arrangement which enables miniaturization. As shown in FIGS. 9 and 10, a sleeve 24 has radial dynamic pressure generating grooves 10 formed on its inner peripheral surface and is covered with a bracket 26. A radial hydrodynamic bearing is formed between the inner peripheral surface of the sleeve 24 and an outer peripheral surface of a shaft 1. A thrust dynamic pressure generating groove 11 is formed on an upper surface of a shoulder portion of the bracket 26. A thrust hydrodynamic bearing is formed between the upper surface of the shoulder portion of the bracket 26 and a lower surface of a hub 4. Further, a lubrication oil as a working fluid is filled between the inner peripheral surface of the sleeve 24 and the outer peripheral surface of the shaft 1 and between the upper surface of the shoulder portion of the bracket 26 and the lower surface of the hub 4, covering at least portions which form the radial hydrodynamic bearing and the thrust hydrodynamic bearing. In this structure, communicating holes 25 for communication between an outer peripheral surface and a lower surface of the sleeve 24 and between an inner peripheral surface and an upper surface of the bracket 26 such that a lubrication fluid 31 can flow therethrough are formed. In this way, a pressure difference can be compensated through the communicating hole 25 even when the pressure difference is generated in the lubrication fluid 31 which is held between the inner peripheral surface of the sleeve 24 and the outer peripheral surface of the shaft 1 between upper and lower ends in an axial direction due to an error in working and the like of the dynamic pressure generating grooves and other components provided in the radial bearing portion. Thus, bubbles due to a negative pressure in the lubrication fluid 31 and the case of excessive floating of the rotor can be suppressed. In the case where the sleeve 24 and the bracket 26 are integrally formed, forming the communicating hole 25 is difficult since it is necessary to form a narrow and long hole with drilling or the like. However, if the sleeve 24 and the bracket 26 are separate pieces as in the above-described structure, it is easy to form a communicating hole because a groove provided on the peripheral outer surface of the sleeve 24 or the inner peripheral surface of the bracket 26 can serve as a communicating hole when the sleeve 24 and the bracket 26 are assembled.
Japanese Laid-Open Publication No. 2004-52931 discloses a structure shown in FIGS. 11 and 12. In the structure, upper and lower surfaces of a sleeve 27 in an axial direction are covered with a bracket 26 and a communicating hole 28 is formed into a squared-c shape.
Japanese Laid-Open Publication No. 2004-239387 discloses a structure shown in FIGS. 13 and 14. In the structure, a flanged sleeve 29 has radial dynamic pressure generating grooves 10 and thrust dynamic pressure generating grooves 11 respectively formed on its inner peripheral surface and an upper surface of a flange portion. The flanged sleeve 29 is covered with a bracket 26. A radial hydrodynamic bearing is formed between the inner peripheral surface of the flanged sleeve 29 and an outer peripheral surface of a shaft 1. A thrust hydrodynamic bearing is formed between the upper surface of the flange portion of the flanged sleeve 29 and a lower surface of a hub 4. In this structure, communicating holes 30 are formed between an outer peripheral surface of the flanged sleeve 29 and a lower surface of the flange portion and between an inner peripheral surface of the bracket 26 and an upper surface of a shoulder portion.
However, spindle motors having conventional structures disclosed in Japanese Laid-Open Publication Nos. 2004-19705 and 2004-52931 have following problems. Since they do not have a communicating hole formed in the thrust bearing portion, a pressure difference is not compensated when the pressure difference is generated between inner and outer peripheral portions of the thrust bearing due to an error in working and the like of the thrust dynamic pressure generating grooves 11 and other components. Thus, bubbles due to a negative pressure in the lubrication fluid 31 and the case of excessive floating of the rotor cannot be suppressed. Further, since thrust dynamic pressure generating grooves are formed on the upper surface of the shoulder portion of the bracket 26 which has a very small area, it is difficult to improve a surface shape of the upper surface of the shoulder portion and a processing precision of the thrust dynamic pressure generating groove. For similar reason, thrust dynamic pressure generating grooves cannot be formed by inexpensive methods such as pressing. This makes difficult to reduce the processing cost.
On the other hand, in the spindle motor having the conventional structure disclosed in Japanese Laid-Open Publication No. 2004-239387, the communicating holes 30 are formed between the outer peripheral portion of the thrust bearing and the lower portion of the radial bearing by employing the flanged sleeve 29. Thus, even when a pressure difference is generated due to an error in working and the like of the thrust dynamic pressure generating grooves 11 and other components, bubbles due to a negative pressure in the lubrication fluid 31 and the case of excessive floating of the rotor can be suppressed. Moreover, since the flanged sleeve 29 is employed, the thrust dynamic pressure generating grooves can be formed easily by inexpensive methods such as pressing.
However, as shown in an enlarged view of a bearing opening in FIG. 14, the communicating hole 30 is opened near the taper portion 13. When the lubrication fluid 31 flows from the lower portion of the radial bearing to the outer peripheral portion of the thrust bearing, not all of the lubrication fluid 31 can enter the thrust bearing. The lubrication fluid 31 tends to leak from a seal formed by the taper portion 13 and a cylindrical wall portion 14.